now-dragging

This property indicates whether the split-bar is currently being dragged
interactively by the end user, returning true if that is so
and nil otherwise. The value should not be
set by an application, but the value could be read in
an on-change function, for
example, to determine whether the new value is an intermediate value
during a drag, rather than a final value at that is being set
programmatically or at the end of a drag.

It could also be called in an on-redisplay function to draw
the split-bar differently
while it is being dragged.